Transcript Lecture 3 - Electrical and Computer Engineering

Part II Data Communications
Chapter 3 Data Transmission
• Concept & Terminology
• Signal : Time Domain & Frequency Domain Concepts
• Signal & Data
• Analog and Digital Data Transmission
• Transmission Impairments & Attenuation
• Nyquist Signaling Rate & Shannon Channel Capacity
Spring, 2003
EE 4272
Spring, 2003
EE 4272
Concept & Terminology
• Direct link: No intermediate devices (except amplifiers, or
repeaters)
• Medium


Guided medium: e.g. twisted pair, coaxial cable , optical fiber
Unguided medium: e.g. air, water, vacuum
• Simplex

One direction: e.g. Television
• Half duplex

Either direction, but only one way at a time
• Full duplex

Both directions at the same time: e.g. telephone
Spring, 2003
EE 4272
Electromagnetic Signal :
Frequency, Spectrum and Bandwidth
• Time domain concepts

Continuous signal:Various in a smooth way over time
 Discrete signal
- Maintains a constant level then changes to another constant level


Periodic signal: Pattern repeated over time
Aperiodic signal: Pattern not repeated over time
Spring, 2003
EE 4272
Periodic Signals
Sine Wave
• Peak Amplitude (A)

maximum strength of signal
• Frequency (f)




Rate of change of signal
Hertz (Hz) or cycles per
second
Period = time for one
repetition (T)
T = 1/f
Spring, 2003
EE 4272
Frequency Domain Concepts
• The electromagnetic signal
usually made up of many
frequencies
• Components are sine waves
• Can be shown (Fourier
analysis) that any signal is
made up of component sine
waves
• Addition of Frequency
Components
• Can plot frequency domain
functions
Spring, 2003
EE 4272
Frequency Domain
• Time-domain function:
s(t)
• Frequency-domain
function:
S(f)
Spring, 2003
EE 4272
Spectrum & Bandwidth
• Spectrum: range of frequencies contained in signal
• Absolute bandwidth: width of spectrum
• Effective bandwidth
 Narrow band of frequencies containing most of the energy
• DC Component: Component of zero frequency
Spring, 2003
EE 4272
Data Rate, Bandwidth & Data Transmission
Data Rate and Bandwidth
• Any transmission system has a limited band of frequencies
-> bandwidth; This limits the data rate that can be carried
Data Transmission
• Data: Entities that convey meaning


Analog: Continuous values within some interval. e.g. sound, video
Digital: Discrete values. e.g. text, integers
• Signals: Electric or electromagnetic representations of data


Analog Signal: continuously varying electromagnetic wave via various
media, such as wire, fiber optic, space
Digital Signal: Use two DC components
• Transmission:Communication of data by propagation and
processing of signals
Spring, 2003
EE 4272
More on Signals & Data
• In a comm. sys., data are propagated from one point to
another by means of electric signal.
• Usually use digital signals for digital data and analog
signals for analog data transmission
• Can use analog signal to carry digital data: Modem
• Can use digital signal to carry analog data: Compact
Disc audio
Reading assignment: p86-89
Spring, 2003
EE 4272
Analog Signals Carrying Analog & Digital Data
Spring, 2003
EE 4272
Digital Signals Carrying Analog and Digital Data
Data
(e.g. Compact Disc Audio)
(e.g. DSL)
Spring, 2003
EE 4272
Analog & Digital Transmission
• Analog Transmission




Analog signal transmitted without regard to content
May be analog or digital data
Attenuated over distance
Use amplifiers to boost signal; Also amplifies noise->distort
• Digital Transmission

Concerned with content
 Integrity endangered by noise, attenuation etc.
 Repeaters used : Repeater receives signal -> Extracts bit
pattern -> Retransmits
 Distortion is overcome since noise is not amplified
Spring, 2003
EE 4272
Advantages of Digital Transmission
• Digital technology

Low cost LSI/VLSI technology
• Data integrity: use repeater rather than amplifier

Longer distances while maintain data integrity
• Capacity utilization


High bandwidth links are economical
High degree of multiplexing easier with digital techniques
• Security & Privacy

Encryption
• Integration: e.g., voice, video, and digital data

Can treat analog and digital data similarly
Spring, 2003
EE 4272
Transmission Impairments & Attenuation
Transmission Impairments
• Signal received may differ from signal transmitted


Analog - degradation of signal quality
Digital - bit errors
• Caused by

Attenuation and attenuation distortion
 Delay distortion: the velocity of the propagation of a signal through a guided
medium varies with frequency
 Noise
Transmission Attenuation
• Signal strength falls off with distance
• Depends on medium
• Received signal strength:

must be enough to be detected
 must be sufficiently higher than noise to be received without error
Spring, 2003
EE 4272
Noise
Additional signals inserted between transmitter and receiver
• Thermal



Due to thermal agitation of electrons
Uniformly distributed across the frequency spectrum
Refer as White noise
• Intermodulation

Signals that are the sum or difference of original frequencies of the
signals sharing a medium
• Crosstalk: A signal from one line is picked up by another

due to electrical coupling between nearby twisted pairs, etc.
• Impulse: Short duration; High amplitude

Irregular pulses or spikes: e.g. External electromagnetic interference
Spring, 2003
EE 4272
Nyquist Signaling Rate & Shannon Channel Capacity
• Channel Capacity: Maximum rate at which data can be transmitted over a
path/channel, under given conditions
• Data rate: Rate (in bps) at which data can be communicated
• Bandwidth B: bandwidth of the transmitted signal is

Constrained by transmitter and medium (in Hertz)
• Nyquist Signaling Rate: The fastest rate at which pulses can be
transmitted into the channel (refer to “digital communication” ?)


For binary signaling: r = 2B Pulse/second
For multilevel signaling: r = 2B log2 M Pulse/second
• Shannon Channel Capacity : maximum achievable bit rate at which
reliable comm. is possible over a channel of bandwidth B & of a given SNR

C = B log2 (1+SNR)
Spring, 2003
refer to “Information Theory”
EE 4272